1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor formed through a back side process.
2. Description of the Prior Art
As the development of electronic products such as digital cameras and scanners progresses, the demand for image sensors increases accordingly. In general, commonly used image sensors are nowadays divided into two main categories: the charge coupled device (CCD) sensors and the CMOS image sensors (CIS). Primarily, CMOS image sensors have certain advantages of low operating voltage, low power consumption, and property of random access. Furthermore, CMOS image sensors can currently be integrated in semiconductor fabrication processes. Based on those benefits, the application of CMOS image sensors has increased significantly.
The CMOS image sensor separates incident light into a combination of light beams of different wavelengths. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. The light of different wavelengths is received by respective optically sensitive elements such as photodiodes and is subsequently transformed into digital signals of different intensities. Thus, it can be seen that a monochromatic color filter array (CFA) must be set above every optical sensor element for separating the incident light.
In conventional process, a CMOS image sensor has a binding pad formed on the surface to connect other components, the manufacturing method of the bonding pad includes: forming an opening to expose the metal trace, filling a conductive material in the opening to form the bonding pad, and after the bonding pad is formed, forming a color filter array and a micro lens. As shown in FIG. 1, which is a schematic drawing illustrating a conventional image sensor. A CMOS image sensor 10 has a dielectric layer 12 and a metal trace 14 on its surface, an opening 16 is formed to expose the metal trace 14, and a conductive layer 18 is then filled in the opening 16 to form a bonding pad and to connect other components. Besides, an insulating layer 21 may be selectively formed on the sidewall of the opening 16 to provide shielding effect and prevent the current flowing through the bonding pad from influencing other components.
Afterwards, a color filter array 36 is formed on the dielectric layer 12 through a spin coating process; a planarizing layer 38 is then formed on the color filter array 36, and a plurality of micro-lenses 40 corresponding to the color filter array 36 are formed on the planarizing layer 38. However, there is an issue in conventional processes: as mentioned above, the color filter array 36 is formed on the dielectric layer 12 through spin coating after the opening 16 is completed. It is worth noting that the existence of the opening 16 has the color filter layer stored up in the opening 16, which is then spun out from the opening 16 during the spin coating step, thereby affecting the uniformity of a thickness of the color filter layer, and causing striation. Striation is disadvantageous to the pixel performances since it affects, for example, the saturation of the CMOS image sensor 10.